package main

// In this example we'll look at how to implement a worker pool
// using goroutines and channels.

import (
	"fmt"
	"time"
)

// here's the worker, of which we'll run several concurrent instances.
// These workers will receive work on the jobs channel and send the
// corresponding results on results. We'll sleep a second per job to
// simulate an expensive task.
func worker(id int, jobs <-chan int, results chan<- int) {
	for j := range jobs {
		fmt.Println("worker", id, "started job", j)
		time.Sleep(time.Second)
		fmt.Println("worker", id, "finished job", j)
		results <- j * 2
	}
}

func main() {
	// In order to use our pool of workers we need to send them work
	// and collect their results. We make 2 channels for this.
	const numJobs = 5
	jobs := make(chan int, numJobs)
	results := make(chan int, numJobs)

	// This starts up 3 workers, initially blocked because there are no
	// jobs yet.
	for w := 1; w <= 3; w++ {
		go worker(w, jobs, results)
	}

	// Here we send 5 jobs and then close that channel to indicate
	// that's all the work we have
	for j := 1; j <= numJobs; j++ {
		jobs <- j
	}
	close(jobs)

	// Finally we collect all the results of the work. This also ensures
	// that the worker goroutines have finished.
	for a := 1; a <= numJobs; a++ {
		<-results
	}
}
